None.
1. Field of the Invention
The present invention generally relates to medical devices, and more particularly relates to devices for reinforcement of a portion of a vascular wall.
2. Description of the Prior Art
It is well known in the prior art to design and build apparatus for the treatment of various vascular disorders. It is common to group these therapies in accordance with the location within the body of the vessel(s) to be treated. For example, peripheral vascular therapies treat vascular disease of the extremities. Similarly, cardiac vascular therapies treat vascular disease of the coronary system. This method of differentiation is particularly helpful in that the medical procedures and corresponding medical devices tend to be specifically tailored to the individual application.
The primary disease effecting the coronary system involves the build-up of material within the lumen of a vessel which partially or completely occludes the vessel preventing adequate perfusion. Though there are reported attempts in the literature to provide treatment using systemic drugs, the primary therapies involve invasive procedures.
Perhaps most common, is the by-pass surgical procedure. Most typically this involves a complete tracheotomy during which those sections of the coronary arteries which are partially or completely occluded are surgically removed. If the occluded sections prove to be quite long, it may be necessary to supply artificial or organic graft material. Common artificial grafts are made from woven polymer fibers. Natural grafts may be transplanted from a human or animal donor or may be harvested from the patient, as with the use of the patient""s saphenous vein.
The key alternative to by-pass surgery is a less invasive procedure termed percutaneous translumenal coronary angioplasty (PTCA). In this procedure, a catheter is inserted percutaneously into an artery (usually the femoral artery in the leg) and advanced so that the distal portion, containing an inflatable balloon reaches the occluded section of the coronary artery. Inflation of the balloon compresses the occluding material into the vessel wall, thus increasing the effective cross section of the vessel. Because the procedure is much less invasive than by-pass, it is much less costly and much less traumatic.
However, the prevalent medical concern about PTCA involves the restenosis rate. A number of preliminary studies have shown that the rate at which treated vessels subsequently reocclude may be unacceptably high. The exact mechanism whereby restenosis occurs is not well understand, notwithstanding considerable on-going research on the topic. Yet, it seems rather intuitive that the vessel wall, in the region of the initial lesion, may have been weakened by the disease. It also seems likely that such a weakened vessel wall may indeed be further weakened by the PTCA procedure, itself.
Thus, it has become a common practice to supplement the PTCA procedure with the implantation of a stent to provide reinforcement of the vessel wall. A stent is a generally cylindrical structure which fits snugly the inside dimension of the inner vessel wall, providing additional radial strength against restenosis. U.S. Pat. No. 4,307,723, issued to Finney, describes a stent having a considerably different configuration which is commonly utilized within the urinary tract.
U.S. Pat. No. 5,989,207, issued to Hughes, shows a relatively elongate stent structure. A stent more specifically configured for coronary use is seen in U.S. Pat. No. 5,879,370, issued to Fischell et al. Frantzen, in U.S. Pat. No. 5,718,713, describes a stent structure produced of a mesh having a flattened outer surface. U.S. Pat. No. 5,843,172, issued to Yan describes a stent which is porous to provide chronic release of a drug. With all of these proposed stent structures, there remain the concerns of accomplishment of the basic purpose of the stent implantation, without undue chronic movement and without undue prevention of the perfusion of the endothelial cells of the stented vessel wall.
The present invention overcomes many of the disadvantages found in the prior art by offering a method of and apparatus for providing the desired stent functions yet having improved properties of placement and chronic implantation. These improvements are derived from the fabrication techniques and physical configuration of the stent of the present invention.
The present invention offers a greater opportunity for positioning and dispensing of medication for chronic drug therapy. It also provides enhanced chronic retention, improved perfusion, and differential flexibility in placement.
In the preferred mode of practicing the present invention, the stent is fabricated from hollow, cylindrical, tube-like stock of a biocompatible material, such as titanium or medical grade stainless steel. The raw stock is preferably xe2x80x9cmachinedxe2x80x9d on a machine tool having a rotary laser cutting head, which cuts a mesh-like pattern through the wall of the metal stock. It may be appropriate, in certain other applications, to utilize a memory-type metal, such as Nitinol.
In addition to the pattern cut entirely through the stock, one or more pockets or channels are cut into but not through the stent wall. These pockets or channels can greatly enhance the positional stability of the stent, because the channel edges tend to more tightly grip the vessel wall. The pockets or channels can incidentally provide a path for blood flow between the outer stent wall and the endothelial cells of the inner vessel wall. As a result, the present invention can provide enhanced perfusion of the stented vessel wall. This enhanced perfusion would especially benefit stent designs having an embedded medication, because it would generate greater distribution of the drug.
The pockets or channels may be machined circumferentially or longitudinally with respect to the normal blood flow. Longitudinal orientation would tend to provide the greatest enhancement of perfusion. This will be the configuration for many applications.
Circumferential orientation of the pockets or channels provides all of the above described benefits and also imparts differential flexibility along the length of the stent. This occurs because the reduced metal of the pocket or channel more readily permits a bend to occur at the location of the channel. This differential flexibility is particularly useful in effective placement of the stent, although it is also helpful in positional retention around bends in the vessel wall.
One particular embodiment of the present invention provides for differentially machining a stent mesh pattern at one or more selected locations. These breaks are useful for imparting a particular desired degree of differential flexibility.